Micromechanical sensing devices are known which sense external movement through changes in the motion of an excited sensing element. An example of such a sensing device is an angular rate sensor, used in applications such as rollover detection for vehicles.
The necessary positioning of components in a device such as an angular rate sensor is such that the excitation of the resonant structure which comprises the sensing element has to be driven in a plane normal to the desired plane of the excited motion. In an angular rate sensor, excitation electrodes must excite the sensing element of the sensor to a vibration mode which is essentially a motion parallel to the surface of the excitation electrodes. This means that the device must be designed so that the excitation motion has components in a direction normal to the desired plane of the excited motion.
A sensing element of such a sensor comprises one or more masses attached to a beam. Current devices achieve the desired excitation characteristics via beams with a specially designed geometry. The cross-section of the beam is created to be asymmetric and has a geometry such that its principal axis is not parallel to the surface normal of the mass (see FIG. 1). Therefore, the beam has a tendency to bend out of the surface. The result is that an excitation mode has small components out of the surface plane of the mass and hence can be driven electrostatically from the side of the plane of the mass.
A problem with this approach is that, due to the relatively small sizes of the beams, process tolerances influence the properties and principal axis of the beam, process tolerance control is therefore a major issue in the fabrication process. This is also the case for other micromechanical devices using resonant structures.